1. Field of the Invention
The present invention relates in general to a method of digitally encoding synthetic speech, and more particularly to a Line Spectrum Pair (LSP) scheme that encodes the LSP synthetic speech parameters using Differential Quantization.
2. Description of the Related Art
In the past several years, semiconductor manufacturers have developed many synthetic speech chips for a great number of applications, including toys, personal computers, car electronics, etc. In these chips the PARCOR algorithm and ADPCM algorithm have been widely used. These well known speech analysis-synthesis methods encode the speech parameters with pulse-code modulation (PCM). PCM is a modulation method in which the peak-to-peak amplitude range of the signal to be transmitted is divided into a number of standard values, each value having its own three-place code. Thereafter, each sample of the signal is transmitted as the code for the nearest standard amplitude. The PCM encoding method encodes each speech sample directly, thereby creating a large number of data bits. Therefore, a speech synthesis chip that encodes the speech parameter using the PCM method will have a large device scale.
Another drawback of the PARCOR algorithm is its bit rate limit, wherein below approximately 2,400 bps the synthesized voice becomes unclear and unnatural.
To overcome the disadvantages of the above synthetic speech algorithms, the LSP method was developed. LSP, an improved algorithm derived from PARCOR, requires only 60% of the bit rate required for PARCOR synthesis, yet still maintains the same level of quality. Since the bit rate needed to perform the operations is lower, the resulting tone is improved. See "Digital Speech Processing Synthesis and Recognition", Sadaok & Furnin, ISBN 0-8247-7965-7, pages 126, 133.